Age of M13: 14 billion years. Mass of stars leaving the main-sequence ~0.8 solar masses

Main Sequence

Sub-giants

Giants

Helium core-burning stars

Here is a way to think about it.

Outside of star

Plenty of hydrogenShrinking core

Where core use to be. And where conditions were right for fusion.

Result…

• Because the core is shrinking there is hydrogen that is introduced into the area around the core where temperatures and pressures are high enough for hydrogen fusion to take place.

• Hydrogen begins to fuse into helium, in a shell around the shrinking helium core.

• Now there are two energy sources in the star.

Two energy sources.

• Gravitational potential energy is being used to make radiant energy in the core.

• The shell around the core is producing energy from the fusion of hydrogen.

• The result of all this energy is that the outer envelope of the star expands enormously. The star becomes a red giant. (luminosity class III)

Age of M13: 14 billion years. Mass of stars leaving the main-sequence ~0.8 solar masses

Main Sequence

Sub-giants

Giants

Helium core-burning stars

Helium core burning

• The core contraction and hydrogen shell burning until at last the temperature is high enough in the core to begin helium fusion. This is around 100 million degrees.

• When this happens the star is fusing Helium into Carbon in its core, and still is fusing Hydrogen into Helium is a shell around the core.

The triple alpha process

Core and shell burning produces more energy than the star produced on the main sequence so the He-

core burning stars are more luminous than when they were main-sequence stars.

Main Sequence

Sub-giants

Giants

Helium core-burning stars

Helium gone in the core

• Helium fusion rate is much faster than the Hydrogen fusion rate was. Within a few hundred million years the supply is gone in the core.

• The core once again shrinks, releasing gravitational potential energy.

• The material in a shell closest to the core begins to fuse helium into carbon, in bursts, as the temperature increases.

• Above this shell, hydrogen is being converted into helium.

Here is a way to think about it.

Outside of star

Plenty of hydrogenShrinking core

Hydrogen shell burning

Helium shell burning

Three energy sources.

• At this point there are three sources of energy in the star, the shrinking carbon core, and two shells.

• The star rapidly expands and heads back up to the giant stage. This is called the asymptotic giant branch, because it asymptotically approaches the red giant branch.

Asymptotic Giant branch phase

• During this phase the helium core burning is not stable. It rapidly turns on and off in bursts. Small explosions.

• The results of these explosions is to eject shocks into the outer envelope of the star. Material in the envelope is lifted off the star, over and over again.

• When the carbon core can no longer contract, everything stops.

• The lost envelope becomes an expanding planetary nebula.

• The exposed carbon core is a white dwarf star.

Planetary nebula & White Dwarf

White Dwarf star

Summary of evolution of lower mass stars• Star is on main-sequence – Core converting hydrogen

into helium.• Star is a Sub-giant -- Core is contracting releasing

gravitational potential energy• Star is a Giant (III) – Core is contracting releasing

gravitational potential energy and hydrogen into helium in a shell around the core.

• Helium core burning phase – Star is converting helium into carbon in the core and hydrogen into helium in a shell.

• Asymptotic Giant branch phase – Core is contracting releasing potential energy, Helium into Carbon in a shell, and hydrogen into helium is a shell around Helium shell.

Notice a pattern

• Whenever a star has an inert core that is shrinking, the star is moving up the giant branch. The star grows in radius

• Whenever there is nuclear fusion in the core the star shrinks back down. Smaller radius.

• This will be important in high mass stars.

Inert core

Core fusing elements

Low mass stars cannot fuse Carbon

• Core temperature is too low to fuse Carbon into other elements.

• The core shrinks until all the free electrons are trapped in spaces between the Carbon nuclei. They set up energy levels and the core acts like a giant atom. Core cannot shrink any more.

• The core is similar in size to the radius of the Earth, but has a mass of as high as 1.4 times the Sun’s mass.

• From here on the core will just slowly cool off. Like a hot piece of metal, slowly cools down.

Planetary Nebula

• During the Helium shell burning phase, there are helium flashes occurring. The helium in the shell doesn’t “burn” at a constant rate. It burns in spurts. Each time helium shell burning turns on, there is an eruption.

• The result is the outer envelope of the star gets shocked, over and over. The outer shell is lifted off in layers.

• The result is a planetary nebula. The exposed Carbon core is a white dwarf.

The Ring nebula – M57

Cat’s Eye Nebula

M57 through a small telescope

Boomerang Nebula

Butterfly Nebula – Central White Dwarf has T = 250,000 K.

Cat’s Eye in optical and X-ray light

Eskimo Nebula

NGC 2440 – Central White Dwarf has T = 200,000 K

Ring Nebula – Multiple mass ejections.

The planetary nebula phase is short lived.

• The radius of a typical planetary nebula is about 1 light year.

• The gas is glowing, so we see an emission nebula.

• Typical elements in at planetary nebula are hydrogen, helium, carbon, oxygen and nitrogen. Also some neon present.

Spectrum of Ring nebula

Sirius – The Dog Star

Sirius is a binary star

Sirius A

Sirius B

Which star is older in this binary system

1 2 3

33% 33%33%1. Sirius B because it is

already a white dwarf2. Sirius A because it is

more luminous3. They are in a binary

system, they must be the same age

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Which star was originally the most massive?

1 2 3

33% 33%33%1. Sirius B because it is

a white dwarf 2. Sirius A because it is

more luminous3. They formed at the

same time so they must have the same mass

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Globular cluster M 4

• The stars with masses higher than about 0.8 solar masses have died.

• There should be a lot of white dwarfs in the a globular star cluster.

White Dwarfs in M 4

• White dwarfs are just the leftover core of the star. It is not producing energy. It is simply cooling off.

• As a WD cools it becomes less luminous because the temperature is decreasing.

• The cooling follows a very simple cooling relation that depends primarily on time. The older the white dwarf, the cooler it is.

• There is a cutoff in the WD temperature. No WD are found that are cooler than the cut off.

Cooling

Cut-off

Why is there a cut-off in the white dwarf population?

1 2 3 4

25% 25%25%25%1. Cooler WD are impossible to detect

2. At a certain temperature, WD explode

3. The universe isn’t old enough to have cooler white dwarfs

4. WD come into temperature equilibrium with the universe and remain that temperature

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Age of the universe using WD cooling

• To date hundreds of thousands of White Dwarfs have been observed.

• There is a temperature cut-off beyond which no white dwarfs are found.

• This is because there hasn’t been enough time since the start of the universe for WD to cool any further.

• The age of the universe computed from WD cutoff is about 12 billion years.

The Death of High Mass Stars

• When a high mass star runs out of hydrogen in its core, the core begins to shrink. The outside of the star expands and the star moves right on the H-R diagram.

• The temperature is cooling and the radius is growing, but the luminosity is virtually constant.

• Since L = σT4(4πR2); T4 must be changing at the same rate as R2

• The star becomes a supergiant (luminosity class I star)

The Death of High Mass Stars

• As the star tracks to the right for the first time the inert helium core is contracting and hydrogen shell burning is occurring.

• At the farthest right, helium core burning begins, converting helium into carbon. And still hydrogen shell burning.

• The star begins to move to the left on the H-R diagram.

The Death of High Mass Stars

• When the helium runs out in the core, the core begins to contract again, there is helium shell burning into carbon, and hydrogen shell burning into helium.

• The star moves right again, toward cooler temperatures and larger radii.

The Death of High Mass Stars

• Finally the carbon core is hot enough to fuse carbon into oxygen and nitrogen.

• The star moves back to the left on the H-R diagram. There is a core changing carbon into oxygen and nitrogen, a shell changing helium into carbon, and a shell changing hydrogen into helium.

A rule of thumb.

• Every time a high mass star moves to the right (cooler temp) on the H-R diagram, the core is inert, but contracting.

• Every time a high mass star moves to the left, the core is fusing one element into another.

• Throughout all of this there is shell burning going on.

Final stage.

• The core of the high mass star fuses:• hydrogen into helium• helium into carbon• carbon into oxygen and nitrogen• oxygen and nitrogen into sulfur and silicon• And finally silicon into IRON.

• At last the core is iron. This is where everything stops with a bang!

The final core and shells of a high mass star

Fusing Iron does not release energy.